Polymerization of p-hydroxymethylbenzoic acid (p-HMBA) affords poly(p-methylenebenzoate), a polymer which exhibits excellent impact properties. The known processes for production of p-hydroxymethylbenzoic acid frequently are cumbersome and run through several stages. For example, p-hydroxymethylbenzoic acid has been prepared by free-radical bromination of p-toluic acid to p-bromomethylbenzoic acid, hydrolysis with aqueous barium hydroxide and subsequent purification by recrystallization from water. Other methods for the preparation of p-hydroxymethylbenzoic acid and/or methyl poly(p-methylenebenzoate) have since been discovered, including:
(a) Hydrolysis of p-toluic acid and derivatives functionalized at the benzylic position, such as p-halomethylbenzoic acid and esters.
(b) Hydrolysis of p-halomethylbenzonitriles, p-hydroxymethylbenzonitrile and p-chloro-toluyl chloride.
(c) Oxidation of p-xylene and substituted p-xylenes, such as p-hydroxymethyltoluene, p-acetoxymethyltoluene and p-xylenediol, and oxidation of p-toluic acid, p-tolualdehyde, and derivatives.
(d) Chloromethylation of benzoic acid and toluene derivatives.
(e) Carboxylation of p-halotoluene compounds via lithium salts.
(f) Disproportionation of terephthaldehyde (Cannizzaro reaction).
(g) Polarographic reduction of dimethyl terephthalate.
(h) Electrochemical reduction of terephthalic acid in aqueous solution.
Of the above methods for preparation of p-hydroxymethylbenzoic acid, the electrolysis of terephthalic acid offers a route to the desired product in one step in good yield and with good selectivity to p-hydroxymethylbenzoic acid.
However, the electrolysis of terephthalic acid requires a catholyte containing a basic solvent diluted suitably to maintain a weak basic condition or an aprotic solvent to which a source of protons has been added. In a suitable method of operation, the catholyte consists of a solvent, preferably water and terephthalic acid with a soluble ammonium salt and ammonia. Reference is made to German Pat. No. 2,642,496 and to commonly-assigned U.S. applications Ser. Nos. 319,120 and 358,222, incorporated herein by reference. The product of electrolysis of terephthalic acid wherein the catholyte contains a soluble ammonium salt and ammonia is the ammonium salt of p-hydroxymethylbenzoic acid which is thereupon acidified. Terephthalic acid and other impurities are typically present in the product.
In more detail, after the electrolysis, German Pat. No. 2,642,496 teaches the electrolysis product can be evaporated to one-half the volume to a concentrate which is then acidified with a mineral acid, especially sulfuric acid, at a temperature of from 5.degree. C. to 15.degree. C., to precipitate the reaction product. The reaction product is washed with water and dried.
Commonly assigned Ser. No. 319,120 teaches that at the end of the electrolysis, the p-hydroxymethylbenzoic acid is isolated from the electrolyte containing ammonium salts by acidifying the catholyte and filtration at a temperature of from 75.degree. C. to 100.degree. C. to remove terephthalic acid. The mother liquor is concentrated under reduced pressure, then cooled to a temperature preferably below 25.degree. C. to obtain p-hydroxymethylbenzoic acid. The impurities in the p-hydroxymethylbenzoic acid obtained by the above typical methods often include terephthalic acid, 4-carboxybenzaldehyde, toluic acid, sulfur, and other oxidized sulfurized by-products if sulfuric acid is used. Other mineral acids, if used, will cause similar impurities. The level of impurities renders the p-hydroxymethylbenzoic acid unusable for polymers having an inherent viscosity of at least 0.50 dl/g without further separate isolation steps.
Commonly assigned Ser. No. 358,222 teaches the crude hydroxymethylbenzoic acid is isolated from the catholyte containing ammonium salts by acidification using known means. The crude p-hydroxymethylbenzoic acid is then hydrogenated to remove 4-carboxybenzaldehyde.
The present invented process differs from the processes taught in commonly-assigned Ser. Nos. 319,120 and 358,222 in that the product of electrolysis of the instant invented process is not acidified to obtain the acids produced by the reduction but instead the ammonium salts, the products of the reduction, are catalytically hydrogenated directly after electrochemical reduction to reduce catalytically 4-carboxybenzaldehyde to the ammonium salt of p-hydroxymethylbenzoic acid. The ammonium salt of p-hydroxymethylbenzoic acid is thereupon decomposed by heat to the free acid.
It is, therefore, an object of this invention to provide a process for the production of poly(p-methylenebenzoate) without need of prior isolation procedures to remove terephthalic acid.
This problem can be solved, surprisingly, by using ammonia or an amine salt in the electrochemical reduction of terephthalic acid to produce an amine salt of p-hydroxymethylbenzoic acid after prior hydrogenation to remove 4-CBA. The amine salt is decomposed in a flash decomposition at a suitable temperature to obtain p-hydroxymethylenebenzoic acid for preparation of poly(p-methylenebenzoate) and of copolyesters of poly(p-methylenebenzoate) and poly(alkyleneterephthalate).